(a) Field of the invention:
The present invention concerns a power supply circuit for an electrical apparatus, more particularly for an audio power amplifier.
(b) Description of the prior art:
Power supply circuits intended for deriving dc output from commercial ac power supply for use in amplifiers and like devices, in general, employ a power transformer for set-up or down of the ac power supply voltage or for isolation between commercial ac power supply source and the dc output. For example, FIG. 1 is an illustration showing the circuit arrangement of a conventional power supply circuit. Such circuit diagram has the configuration to enable that the voltage of the ac power supply 1 is set up or down by a transformer 2, and that the voltage of the output of this transformer 2 is subjected to rectification-and-smoothing by a rectifying and smoothing circuit 6 which is composed of diodes 3 and 4 and a capacitor 5, and that the output of this rectifying and smoothing circuit 6 is supplied to a load 7.
In such known power supply circuit, however, an attempt to obtain a large capacity output has tended to an undesirable increase in the size, configuration and weight of the power transformer which is employed. This is because of the fact that an increase in the output, i.e. an increase in the load current, necessitates a power transformer having windings of a diameter large enough to sufficiently sustain such large current flow. Also, in the known power supply circuit described above, the voltage V derived across the secondary winding of the transformer 2, i.e. the rectified and smoothed voltage in the rectifying and smoothing circuit 6, the output voltage V.sub.DC which is supplied to the load 7, and the charging current i.sub.c for the capacitor 5 assume their waveshapes as those shown at (a), (b) and (c) in FIG. 2, respectively. As will be understood from these illustrations, this known power supply circuit is such that the period of time required for the transmission of power supply in the transformer 2 is limited to only that period of time t.sub.2 of flow of the charging current with respect to a half cycle of the voltage V. In other words, for those periods of time t.sub.1 and t.sub.3, there are allowed to flow only exciting currents. Thus, the known power supply circuit has another disadvantage that the transformer 2 has periods of time wherein only the exciting currents are allowed to flow and that these exciting currents are wasteful.
Also, apart from the conventional power supply circuit discussed above, there has been proposed and placed on markets a power supply circuit of another type which is arranged so as to control the power which is to be supplied to a load, by the use of a bidirectional thyristor such as Triac (a trademark of a product of General Electric, Inc. of U.S.A.), as shown in FIG. 3. The power supply circuit shown in FIG. 3 is of the arrangement that, in the power supply circuit arrangement shown in FIG. 1 designed so that the voltage of the ac power supply 1 is supplied to the primary coil of the power transformer 2, and that the voltage derived at the secondary coil of this transformer 2 is rectified and smoothed by diodes 3 and 4 and a capacitor 5 for being supplied to a load 7, there is inserted a bidirectional thyristor 8 between the ac power supply 1 and the primary coil of the transformer 2, and that this bidirectional thyristor 8 is triggered by a trigger circuit which, in turn, is comprised of triggering elements such as a resistor 9, a capacitor 10 and a diac 11. This another type power supply circuit is operative so that, by appropriately setting the values of the resistor 9 and the capacitor 10, and also by controlling the firing angle of the bidirectional thyristor 8, the amount of power supply which is to be provided to the load 7 is determined.
However, this latter power supply circuit also is not free of the inconvenience that, when the bidirectional thyristor 8 is turned "on" and when, thus, a power is supplied from the secondary coil of the transformer 2 to the load 7 side, there is easily allowed a large exciting current to flow to the primary coil of the transformer 2 after the current i.sub.c flowing to the load 7 side has become zero. More particularly, in case the voltage which is applied to the primary side of the transformer 2 of this known power supply circuit is assumed to be that part of voltage shown in the hatched portion of the voltage V.sub.i of the ac power supply 1 at (a) of FIG. 4, the current i.sub.c which is allowed to flow to the load 7 side will become such momentary current as shown at (b) of FIG. 4. Thus, even after this current i.sub.c has dropped to zero, there easily flows to the primary side of the transformer 2 an exciting current of a large amount and having a delay in phase by 90 degrees. Accordingly, in such power supply circuit, there are needed such counter-measures as increasing the density of the magnetic flux of the core and/or increasing the number of turns of the windings of the transformer. Each of these counter-measures to be taken is entailed by an increase in the size and weight of the power transformer, and also by the use of windings of a large diameter from the viewpoint of preventing of burning caused by such large exciting current flow. Thus, in this case, too, the power supply transformer tends to have, undesirably, a further enlarged size and increased weight.
As the related techniques of the prior art in this field of art, it is also known to stabilize the output voltage of power supply circuit shown in FIG. 3 by using feedback technique. This technique has been disclosed in, for example, U.S. Pat. No. 4,051,425, Japanese Layingopen Utility Model Application No. 50-141743, U.S. Pat. No. 3,466,527, U.S. Pat. No. 3,723,849 and U.S. Pat. No. 3,506,905. These prior techniques, however, invariably have the inconveniences and disadvantages similar to those discussed above.